I'm sure there are plenty of others by now. I'm at my second sprite, it's reaching high levels, and I'm running out of materials. I don't want to drain my resources completely though, so I decided to not feed it with any material I have less than 10 of. This means it's on a diet of Soul Dust and Power Stars whenever available.
What do you people find to be the best feeding methods these grim days when our supplies of FSC mats are running dry?
my seraph has a steady diet of heat embers pulled from the still-beating hearts of danger room slimes, zombies, beasts, what have you. my mask(still a baby) and drakon(in the future, maybe) get brute cores.
the only materials I use to feed my sprites are ones that drop from a mission run, and only those. Jelly king is especially profitable for feeding, because rocky cores are plentiful.
Best way to feed, imo, without breaking your inventory, is take the time to see what you got from a run, choose what to feed out of those materials, then stop. at the very least, you will break even.
If you don't care about the trinkets you get from Brinks semi-annually for Spark tokens, you can buy 5* mats from him.
do FSC sometimes, feed my higher level sprite with 3* mats, my lower level with 4* until I have 15, uneven 5* mats, then back to 3* mats. it's not much, but until I have 50 of one 3* mats I won't start to worry. if I'll start, FSC. it gives lots of torchstones.
I just feed them 4* mats. Takes a while, but at least it's affordable.
By the way, the section Heating of the wiki page Battle Sprite has some rough figures for how many materials you might need, in case you don't know that kind of info (I added it only recently).
Feeding em 3* materials is slow but works. I would never dip below 3* though, it basically isn't even noticeable.